Neurons communicate with each other through synapses. Synapses are formed by both a presynaptic neuron and a postsynaptic neuron. During development of the central nervous system (CNS), countless new synapses are established, and new synapses continue to form throughout life. The long-term goal of this application is to determine the molecular basis of synapse formation in the vertebrate CNS. Recently, the first proteins have been identified that initiate the formation of synapses. One is SynCAM, a synaptic cell adhesion molecule that connects pre- and postsynaptic sides. Importantly, SynCAM induces the formation of new, fully functional synapses. It is highly expressed in the developing CNS during intense synaptogenesis, indicating a function for this molecule in synapse formation throughout the brain. The objective of this application is to determine the extra- and intracellular interactions of SynCAM that initiate synapses, and to characterize them further. Achieving these goals is important for human health. Alterations in synapse formation affect synaptic plasticity, which is associated with changes in human behavior, learning and memory, and addiction. Furthermore, deficits of synapse formation and synaptic loss have been suggested for neurodevelopmental and neurodegenerative diseases. To attain the objective of this application, three aims are pursued. First, the role of adhesion and signaling molecules in CNS synapse formation is examined. The experiments involve biochemical characterization of SynCAM and the functional analysis of its extracellular interactions in cultured hippocampal neurons. Second, the intracellular SynCAM interactions driving synaptic membrane specializations are determined. These experiments involve the purification and characterization of intracellular binding partners of SynCAM from brain. Third, the site of SynCAM action in pre- and postsynaptic membrane specialization is analyzed. These studies are not only conducted in cultured neurons, but also in genetically altered mice to manipulate synapse formation in vivo and to test effects on synaptic plasticity and brain development. In summary, this application aims to identify the molecular interactions involved in CNS synapse formation, which will allow testing whether these processes are affected in disorders of the human brain.